It is no secret to the Muslim immigrants of Bay Ridge, Brooklyn, that spies live among them..........................

The trial's revelations, and its outcome for the defendant, Shahawar Matin Siraj, have brought a bitter reckoning among Muslims in the city. Many see the police tactics unveiled in the case as proof that the authorities — both in New York and around the nation — have been aggressive, even underhanded in their approach to Muslims.

And despite the conviction of Mr. Siraj, who was found guilty on all four of the counts he faced, some Muslim leaders remain convinced that he was entrapped, including an imam who knew the informer and had found him to be suspicious.

Police Commissioner Raymond W. Kelly declared the verdict a milestone in the city's fight against terrorism. Muslim leaders say they support efforts to safeguard the country, but many believe that the Siraj case may have set back another battle that the police have been waging: to win their trust and cooperation.

In Bay Ridge, Palestinian, Syrian and Egyptian immigrants have long engaged in their own form of surveillance, trying to discern the spies in their midst. It is a habit imported from the countries they left behind, where informers for the security services were common and political freedoms curtailed.

In the years since Sept. 11, as word of informers spread among the smoky sheesha cafes and tidy mosques of Bay Ridge, a familiar fear has fallen over the neighborhood. It asserts itself quietly, in the hush of conversation and the wary stares that pass between strangers.

"It's like a police state here," said Omar Maged, 34, an assistant teacher at a public high school. "We do not feel that we are living in the most free country in the world."

In the wake of the trial, police officials sought to dispel the notion that they are taking aim at the Muslim community.

Paul J. Browne, the Police Department's chief spokesman, said undercover officers were used only to investigate reports of possible criminal activity. This was the case, he said, with the detective involved in the investigation of Mr. Siraj. The officer had been sent to live in Bay Ridge for two years.

"The notion that he was in there gratuitously observing the Muslim community is false," Mr. Browne said.

The relationship between law enforcement and Muslims has long been fragile.

After Sept. 11, Muslims came under immediate and intense pressure by the authorities. Hundreds of men were detained for questioning and thousands nationwide were placed into deportation proceedings.

Over time, a necessary, if uncomfortable relationship emerged between Muslims and the police watching over them. Efforts were made by both camps to cultivate trust.

"We've been repairing the cracks steadily and gingerly," said Wael Mousfar, the president of the Arab Muslim American Federation.............................

Others complained of what they see as a two-tiered approach by the authorities: on one level it is public, and on another, it is hidden.

"They want to formally be introduced to the community but they don't need to be," Ms. Sarsour said. "They already have their informants among us."

On May 12, in the middle of the trial of Mr. Siraj, Mr. Kelly met with 150 Muslims at a youth center in Bensonhurst, Brooklyn. He showed them a 25-minute video that the Police Department created to train new officers to be sensitive toward Arabs and Muslims. He said he was there to hear their "concerns about issues of public safety," according to a transcript of his speech.

Only after several questions did anyone mention the trial. Debbie Almontaser, a board member of a Muslim women's organization, told Mr. Kelly that she was saddened that the police had resorted to "F.B.I. tactics," and that she thought this was polarizing the Muslim community.

Applause swept the room.

Mr. Kelly told the audience he could not comment on the case.

Whether it will seriously hinder relations between the authorities and Muslims in New York remains to be seen. Some were doubtful.

"This is a chance to enhance our relationship with the police," said Antoine Faisal, the publisher of Aramica, an Arabic and English language newspaper based in Bay Ridge. "These people are being paid to do their job."

An air of suspicion hung over Bay Ridge well before Mr. Siraj was arrested in August 2004. Some people stopped attending the neighborhood's two major mosques, preferring to pray at home. Others no longer idle on the street after work.

"The vibe is not the same anymore," said Omar, 22, a Yemeni immigrant who works at a bookstore and gave only his first name. "We're exposed."

Conversations are often carefully scripted. Several people interviewed said they no longer discussed politics in public.

"When you sit down and politics comes to your head, you think, 'Who's around?' " said Mohammad Gheith, 17, a high school senior who often visits the smoke-filled Meena House Cafe on Bay Ridge Avenue.

Several blocks away, at a grocery store along Fifth Avenue, Mahmoud Masoud said he sensed the presence of informers.

"Sometimes you look a person in the eye, there's a feeling," said Mr. Masoud, 65, a Palestinian immigrant. "You can say anything you want, but don't curse the system. That's what they care about."

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They pulled down the metal gate and locked the front door. It was hours before the store's regular closing time.

"They hate us Muslims," said Mr. Siraj's mother, Shahina Parveen, steadying herself on her husband's arm. "My son is innocent. Eldawoody is criminal," she said, yelling out the last word................................